January 21, 2026 -- For many years, cesium atomic clocks have been reliably keeping time around the world. But the future belongs to even more accurate clocks: optical atomic clocks. In a few years' time, they could change the definition of the base unit second in the International System of Units (SI). It is still completely open which of the various optical clocks will serve as the basis for this. The large number of optical clocks that the Physikalisch-Technische Bundesanstalt (PTB), as a leading institute in this field, has realized could be joined by another type: an optical multi-ion clock with ytterbium-173 ions. It could combine the high accuracy of individual ions with the improved stability of several ions. This is the result of a cooperation between PTB and the Thai metrology institute NIMT. The team led by Tanja Mehlstäubler reports on this in the current issue of the journal Physical Review Letters. The results are also interesting for quantum computing and, with a new look inside the atom, for fundamental research.

Optical atomic clocks with single ions (such as ytterbium-171) are particularly accurate, while clocks with several particles (such as strontium atoms) are very stable. Tanja Mehlstäubler is researching a combination of these two properties and has already realized a multi-ion clock with indium. She is now also looking at ytterbium for the multi-ion idea, albeit a new isotope: ytterbium-173. "This isotope has a particularly interesting transition", explains the physicist.

Transition means the quantum leap in atomics clock: the change of quantum state, which is only possible with a very specific frequency of microwave or laser radiation. Microwave radiation is used for the current cesium atomic clocks. Optical clocks work with laser radiation. Because these oscillations are around a hundred thousand times faster, time can be subdivided more finely and therefore measured more accurately.

The quantum leap in the new ytterbium isotope leads to an excited state with a very long lifetime. "This allows us to make more stable measurements," explains first author Jialiang Yu. "But such transitions usually require strong laser light, which in turn can have major disadvantages." However, this ytterbium isotope has a very specially shaped atomic nucleus and special properties that enabled the team to overcome the problems and even control several ions simultaneously.

This has now paved the way for a multi-ion optical ytterbium clock that combines the high accuracy of single-ion clocks with the improved stability of multi-ion operation. The new atomic species is also very well suited as a multi-qubit for quantum information, as the quantum states can be manipulated extremely precisely using laser radiation.

The new atomic species is also very well suited as a multi-qubit for quantum information, as the quantum states can be manipulated extremely precisely by laser radiation and more quantum information can be encoded simultaneously. This opens up a new possibility for quantum computer research.

Measuring the lifetime of the clock state for the first time provides valuable information about the structure of the atomic nucleus and enables sensitive tests of nuclear physics, for example for possible effects beyond the standard model of physics.

The work was supported by the German Research Foundation DFG (DQ-mat), by the German Excellence Initiative (QuantumFrontiers-390837967) as part of the EU-wide metrology research program (EMPIR project 22IEM01 TOCK) and by the Max Planck RIKEN PTB Center for Time, Constants and Fundamental Symmetries.